32

4.0 THERMAL UPRATING OF EXISTING TRNAMISSION

LINES

4.1 INTRODUCTION

As explained in the previous chapter, HTLS conductors are said to have a great

potential to be a solution in the case of thermal uprating of existing transmission lines.

However, the selection of HTLS conductors have to be accomplished accordingly to

the existing power system. Requirements of each system is unique by country wise

and each country has their own issued to be dealt with. Therefore the use of these

conductors to Sri Lanka’s power system shall be studied based on its unique

requirements.

Transmission lines which require thermal uprating, is studied and identified during the

process of transmission line planning. Usually these studies and identifications are

based on sophisticated computer simulations programmes. In CEB, PSSC software is

used for planning purposes.

Currently there are few lines are identified to be thermally uprated,

1. Athurugiriya - Kolonnawa 132kV Transmission line

2. Pannipitiya - Panadura 132kV Transmission line

3. Pannipitiya - Ratmalana 132kV Transmission line

4. Samanalawewa - Embilipitiya 132kV Transmission line

Single Line Diagram of Sri Lanka’s transmission system is attached in Appendix C.

4.2 ALTERNATIVES TO UPRATE EXISTING TRANSMISSION LINE

• Construction of a new transmission line using a conductor having higher cross section

• Use of existing transmission line towers with a suitable HTLS conductor

after reinforcing towers and foundations if necessary

Average life span of a transmission line can be of 40 to 100 years. This prediction

heavily depends on the environment conditions as well as operation and maintenance

33

process being carried out of that line. Therefore other than the change of conductors,

the stability of structures and foundations shall be considered.

4.3 ALGORITHM FOR SELECTING CONDUCTORS DURING

RESTRINGING

Figure 4.1- Algorithm for Transmission Line Uprating

34

4.3.1 Study of Reconstruction of Existing Line Using Manual Method

To find out the expedience of the proposed algorithm given in Figure 4.1, a real life

example is studied below;

Case 1: Pannipitiya – Ratmalana 132kV Transmission Line

This line is destined to thermally uprated as the existing line capacity is no longer

enough due to the increasing demand of respective areas. Below are some of the basic

details of the line;

Line Name : Pannipitiya – Ratmalana 132kV Trans. line

No. of Circuits : 2

Conductor Type : ACSR, Lynx

Line Length : 7km

Operating Temp. : 15°C (Min) and 54°C (Max)

Table 4.1 - Tower Types and Span length of Pannipitiya-Ratmalana Line

Tower No Tower Type Span (m) 1 TDT + 0 30 2 TD6 + 3 286 3 TD3 + 3 407 4 TDL + 0 302 5 TDL + 3 353 6 TD3 + 3 391 7 TDL + 3 355 8 TDL + 3 366 9 TDL + 0 341

10 TDL + 3 359 11 TDL + 3 393 12 TDL + 0 355 13 TDL + 0 327 14 TDL + 3 322 15 TD3 + 0 340 16 TDL + 0 324 17 TDL + 0 323 18 TDL + 0 340 19 TDL + 3 356 20 TD3 + 0 314 21 TDT + 0 242

35

Availability of ROW

Pannipitiya – Ratmalana transmission line is located in western province of Sri Lanka

which has a very high population density level. This area has been developed a lot

during past few decades and it is very difficult to find unused land areas in the area.

There are lots of constructions have taken place under and alongside the existing line

violating the ROW requirements.

Figure 4.2 - Aerial view of Pannipitiya – Ratmalana line Source: Google Earth

From Figure 4.2, it can clearly be seen that the area is heavily populated under the

power line and there is no ROW for a construction of new transmission line. However

there may be possibilities such as complete demolition of existing line and construction

of new line right over where the existing line was. This solution becomes less

practicable given that the inability of taking such a long interruption period. At the

same time uprooting of existing foundation is a very difficult activity which require

additional time and manpower requirements. Therefore the decision becomes “No” for

the condition “Rural Area” in the algorithm.

Then the next option is to look for restringing of the existing line with newer ACSR

conductor with higher cross section. According to the future transmission planning

36

map of CEB, the requirement of new Pannipitiya - Ratmalana line is to have more than

800 A in a circuit. Therefore new ACSR conductor shall have above CCC to fulfill the

given requirement.

Current Carrying Capacity of Zebra Conductor [16];

Imax Current Carrying Capacity A 834 Outputs

Prad Radiation Heat Loss W 15.3260 Pconv Convention Heat Loss W 55.5927 Psol Solar Heat Gain W/m 14.28138 Nu Nusselt Number 15.920 Re Reynolds Number 788.55 RT Electrical Resistance of the conductor Ω/m 0.00008149 Inputs Ƴ Solar Radiation Absorption Coefficient 0.5 D Conductor Diameter m 0.02862 Si Intensity of solar radiation W/m2 998 s Stefan- Boltzmann Constant Wm-2K-4 5.67E-08

Ke Emissivity coefficient 0.5 T1 Ambient Temperature K 305 T2 Final equilibrium Temperature K 348 λ Air thermal Conductivity Wm-1K-1 0.02585 v Wind Speed m/s 0.5

T25 Minimum Operating Temperature oC 25 T75 Maximum Operating Temperature oC 75 R25 Resistance of the conductor @25 Ω/m 0.06841 R75 Resistance of the conductor @75°C Ω/m 0.08149 Tθ Operating Temperature oC 75

Figure 4.3 - Current Carrying Capacity of Zebra Conductor

Figure 4.3 shows, the current carrying capacity of Zebra conductor based on IEC

61597. According to that ACSR Zebra conductor has the capacity to carry more than

800A in the circuit.

Nonetheless, it is to be checked whether the existing towers are capable of handling

the forces exerted by new conductors because of the fact that the initial design had

been carried out for Lynx conductors which has lesser unit weight and diameter

compared to Zebra conductor. Therefore additional forces exerted by new conductors

are calculated to check the safety of the towers.

37

Checking Tower Strength

Towers are handling three types of forces exerted by conductors [18, 19].

Transverse Forces - Due to Wind Pressure on conductors

Longitudinal Forces - Due to conductor tension

Vertical Forces - Due to conductor weight

These forces are acting right angle to each other.

Wind Span

The wind span is half the sum of the adjacent span lengths of a particular tower.

Transverse force acting on conductor depends on the wind span of towers.

Weight Span

The weight span is the distance between the lowest points on adjacent sag curves on

either side of the particular tower. Weight span can be minus, when towers are sitting

on mountainous terrains.

However, tower strength is usually restricted by transverse loads and longitudinal

loads. Even longitudinal forces become cancelled in suspension towers under normal

operation. Steel is good against axial forces and it is becoming vulnerable under

bending loads. Therefore, towers are capable of absorbing additional vertical forces

but transverse forces.

FTransverse

FLongitudinal

FVertical

Figure 4.4 - Forces Acting on Towers

Plan View of a Tower

38

Figure 4.5 - Wind & Weight Span of Towers

FTransverse = Wind Pressure (N/mm2) x Diameter (m) x Wind Span (m)

Transverse Force exerted by Lynx Conductor,

= 970 N/m2 x 0.01953 m x 360m

= 6820 N

Transverse Force exerted by Zebra Conductor;

= 970 N/m2 x 0.02862 m x 360 m

= 9994 N

% Increase of Transverse Force exerted by New Zebra Conductor;

= (9994−6820)6820

𝑥𝑥 100% = 46.54%

FVertical = Unit Weight (kg/m) x 9.80665 (ms-2) x Weight Span (m)

Vertical Force exerted by Lynx Conductor;

= 0.842 kg/m x 9.80665 ms-2 x 600m

= 4954 N

Vertical Forces exerted by Zebra Conductor;

= 1.621kg/m x 9.80665 ms-2 x 600 m

= 9538 N

% Increase of Vertical Force exerted by New Zebra Conductor;

39

= (9538−4954)4954

× 100% = 92.53%

FLongitudinal = 𝐔𝐔𝐒𝐒𝐔𝐔𝐔𝐔𝐔𝐔𝐑𝐑𝐔𝐔𝐔𝐔 𝐓𝐓𝐔𝐔𝐂𝐂𝐓𝐓𝐔𝐔𝐒𝐒𝐔𝐔 𝐒𝐒𝐔𝐔𝐒𝐒𝐔𝐔𝐂𝐂𝐒𝐒𝐔𝐔𝐒𝐒 (𝐤𝐤𝐤𝐤)

𝐒𝐒𝐑𝐑𝐒𝐒𝐔𝐔𝐔𝐔𝐒𝐒 𝐅𝐅𝐑𝐑𝐅𝐅𝐔𝐔𝐒𝐒𝐒𝐒 (𝟐𝟐.𝟓𝟓)

Maximum Longitudinal Force exerted by Lynx Conductor;

= 78.9/2.5 = 31.4kN

Maximum Longitudinal Force exerted by Zebra Conductor;

= 131.9/2.5 = 52.76kN

% Increase of Longitudinal Forces exerted by New Zebra Conductor;

= (52.76−31.4)31.4

× 100%

= 68%

It is obvious that the safety factors are largely violated if new Zebra conductor is

installed in place of existing Lynx conductors. However most of the towers are not

fully utilized in terms of forces. Due to the restrictions of terrain type and the

availability of ROW, tower spotting has been done in some places where, full tower

utilization is not achieved.

Load adding to an already utilized tower

(a) Adding Vertical Loads

Unused Weight Spans

Figure 4.6 - Unused Weight Span of Towers

40

Unused Weight Span = Tower Designed Weight Span – Actual Weight Span

= (Sweight-Sx)

Addable longitudinal Force (kg) = Unused Weight Span x Conductor Unit to the tower Weight (m) x Number Conductor runs (n)

= (Sweight-Sx) x m x n

Figure 4.6 shows the amount of vertical loads that is addable to the towers which are

not using their fullest designed weight spans.

(b) Adding Transverse Force

Unused Wind Span

Figure 4.7 - Unused Wind Span of Towers (a)

41

Figure 4.8 - Unused Wind Span of Towers(b)

Unused Wind Span = Tower Designed Wind Span – Actual Wind Span

= (Swind - Sx)

Addable vertical Force (kg) = Unused Wind Span x Conductor Diameter to the tower (d) x Wind Pressure (P) x Nos. Conductors (n)

= (Sweight - Sx) x d x P x n

(c) Angle Compensation

Figure 4.9 - Angle Compensation of angle towers

42

2Tmaxsin (α/2) – Swind x P x d = 2TmaxSin (β/2) – Susable x P x d

P - Wind Pressure

D - Diameter of the conductor

Tmax - Ultimate Tension/ Safety factor @ stringent Conn

Table 4.2 - Loads addable to existing towers

# Tower

Type

Span

(m)

Weight

Span

(m)

Wind

Span

(m)

Unused

Weight

Span

(m)

Unused

Wind

Span

(m)

Addable

Vertical

Load

(N)

Addable

Transverse

Load (N)

1 TDT + 0 30 182.36 158.0 67.64 202.0 3364 22993

2 TD6 + 3 286 347.55 346.5 552.45 13.5 27477 1537

3 TD3 + 3 407 321.84 354.5 578.16 5.5 28755 626

4 TDL + 0 302 351.68 327.5 248.32 32.5 12350 3699

5 TDL + 3 353 368.15 372.0 231.85 -12.0 11531 -1366

6 TD3 + 3 391 362.26 373.0 537.74 -13.0 26745 -1480

7 TDL + 3 355 391.35 360.5 208.65 -0.5 10377 -57

8 TDL + 3 366 315.60 353.5 284.4 6.5 14145 740

9 TDL + 0 341 382.05 350.0 217.95 10.0 10840 1138

10 TDL + 3 359 369.25 376.0 230.75 -16.0 11477 -1821

11 TDL + 3 393 336.90 374.0 263.10 -14.0 13086 -1594

12 TDL + 0 355 386.06 341.0 213.94 19.0 10641 2163

13 TDL + 0 327 354.64 324.5 245.36 35.5 12203 4041

14 TDL + 3 322 265.44 331.0 334.56 29.0 16640 3301

15 TD3 + 0 340 305.59 332.0 594.41 28.0 29564 3187

16 TDL + 0 324 413.13 323.5 186.87 36.5 9294 4155

17 TDL + 0 323 211.33 331.5 388.67 28.5 19331 3244

18 TDL + 0 340 435.00 348.0 165.00 12.0 8206 1366

19 TDL + 3 356 340.23 335.0 259.77 25.0 12920 2846

20 TD3 + 0 314 264.50 278.0 635.50 82.0 31607 9334

21 TDT + 0 242 151.00 121.0 99.00 239 4924 27205

Checking Conductor Vertical Loads

Table 4.2 shows, the status of each tower. Calculations are done as explained under

above clause and design wind and weight spans are taken from CEB technical

specifications. Additional transverse and vertical forces that could be absorbed by the

43

towers are calculated with the help of unused wind and weight spans. It is clear that all

the towers are capable of handling additional vertical loads exerted by new Zebra

conductor.

Table 4.3 - Additional Vertical Loads on Towers

Tower No

Used Weight Span (m)

Additional Vertical

Forces = a (N)

Addable Vertical

Forces = b (N)

Tower Strength in terms of Vertical

Forces (a<=b)

1 182 1407 3364 Ok 2 348 2682 27477 Ok 3 322 2484 28755 Ok 4 352 2714 12350 Ok 5 368 2841 11531 Ok 6 362 2796 26745 Ok 7 391 3020 10377 Ok 8 316 2436 14145 Ok 9 382 2949 10840 Ok

10 369 2850 11477 Ok 11 337 2600 13086 Ok 12 386 2980 10641 Ok 13 355 2737 12203 Ok 14 265 2049 16640 Ok 15 306 2358 29564 Ok 16 413 3188 9294 Ok 17 211 1631 19331 Ok 18 435 3357 8206 Ok 19 340 2626 12920 Ok 20 265 2041 31607 Ok 21 151 1165 4924 Ok

Table 4.3 has studied whether additional vertical forces provided by the new conductor

are accommodated by comparing it with addable vertical loads. All the towers are

capable of absorbing the new forces exerted by the new conductor and so there is no

violation of safety in terms of vertical forces in this case.

44

However most of the towers are not capable of absorbing transverse forces that will

exert by the new Zebra conductor. Even in the same design, some of the towers are

loaded more than they are supposed to, in terms of transverse forces. Therefore it is

obvious that safety factors will get violated in a considerable amount with the

replacement of existing conductor with a conductor which has two times the diameter.

Checking Conductor Transverse Loads

Table 4.4 - Additional Transverse Forces

Tower No

Used Wind Span (m)

Additional Transverse

Forces = c (N)

Addable Transverse

Forces = d (N)

Tower Strength in terms of Vertical Forces = (c<=d)

1 158 1393 22993 Ok 2 347 3055 1537 No 3 355 3126 626 No 4 328 2888 3699 Ok 5 372 3280 -1366 No 6 373 3289 -1480 No 7 361 3179 -57 No 8 354 3117 740 No 9 350 3086 1138 No 10 376 3315 -1821 No 11 374 3298 -1594 No 12 341 3007 2163 No 13 325 2861 4041 Ok 14 331 2919 3301 Ok 15 332 2927 3187 Ok 16 324 2852 4155 Ok 17 332 2923 3244 Ok 18 348 3068 1366 No 19 335 2954 2846 No 20 278 2451 9334 Ok 21 121 1067 27205 Ok

From Table 4.4, it is clear that more than 50% of the towers are not capable of

absorbing additional transverse forces exerted by the new conductor.

45

Checking Conductor Longitudinal Loads

Conductor longitudinal loads are acting along the conductor. In line towers or

Suspension towers, the resultant longitudinal force acting on towers is zero as the

similar forces are acting in opposite directions. However in angle towers, the resultant

longitudinal force will not be zero. Therefore with the use of new Zebra conductor, its

maximum UTS cannot be used for tensioning as existing structures are not designed

to withstand that value. If Zebra conductor’s initial tension is selected, the safety factor

of the tower will get violated in a considerable amount.

Tower Designed Strength = 79.8 kN

Typical Zebra conductor’s Initial Tension = 131.9/2.5 = 52.76kN

New safety Factor = 79.8/52.76 = 1.5125 (66%)

Reduction in safety factors = (2.5−1.5125)2.5

× 100% = 39.5%

It can be seen that the safety factor is reduced by almost 40% if the typical Zebra

conductor is used on existing towers. Given the aging factor of towers, the use of an

ACSR Zebra conductor with a higher cross section seems to be an extremely risky job.

According to the above calculations, we could see that the existing towers are not

capable of absorbing the longitudinal forces exerted by Zebra conductor. If we

consider a line that could accommodate those additional transverse forces, still the

longitudinal forces will not satisfy the required safety factors.

In those circumstances, safety can be improved by employing a lower tension on

towers. According to the algorithm given above, once the existing towers are not

providing enough strength, a lower tension value can be used for stringing new

conductors. Although this method will in turn create some ground clearance issues, as

the conductor sag will be increasing at higher temperatures for lower initial tension

values.

Conductor Sag for Lynx conductor at 54°C = 6.7 m (for 300m span)

This is the ground clearance value that had been used during the construction of old

transmission line. Designers might have kept 0.3 m to 0.5 m for surveying and sagging

error. This must be ensured after referring to the existing profile drawing given they

46

are available. Otherwise a simulated design for the initial condition shall be carried out

to finding out whether such a clearance is left. If that is available, that will be an

advantage as the line could tolerate additional sag without violating required ground

clearance.

Therefore, ground clearance that could be achieved by reducing tension of the

conductor is discussed.

Sag of new Zebra conductor at Max. Operating Temperature = 7.52m (Span =300m, Min Tem = 7°C, Max Tem =75°C, Initial Tension = 52.76kN) Ground Clearance could be achieved = Height to the bottom most conductor – Sag

= 13.7m – 7.52m = 6.18m (< 6.7)

Safety Factor Ratio = 79.8/52.76 = 1.51 (<2.5)

Safety Factor % UTS = 100/1.51 = 66.12%

It can be seen that by using typical initial tension of Zebra conductor (52.76kN) would

result a lower safety factor and even the required ground clearance according to the

CEB specifications which is 6.7m, cannot be achieved.

Table 4.5 - Tension vs Ground Clearance

Conductor Longitudinal Force (kN)

Sag @ Maximum Operating

Temperature (m)

Ground Clearance (m)

Safety Factor

Ratio % UTS

52.76 7.52 8.195 1.51 66.12 50 7.96 7.755 1.60 62.66 45 8.85 6.865 1.77 56.39 40 9.91 5.805 2.00 50.13 35 11.22 4.495 2.28 43.86

31.92 12.21 3.505 2.50 40.00

From the Table 4.5, it can be seen that by reducing initial tension to improve safety

factor is not a solution as the ground clearance getting minimized with reduced tension.

47

There are mainly two reasons, that minimized Ground Clearance cannot be used for

the new modifications;

1. There are illegally constructed buildings under the power line, where

by higher sag values will violate required clearance from obstacles.

2. Lower Ground clearance will increase EMF level under the power line,

which would be harmful to human health.

It obvious that first case is violated under the power line where already constructions

have taken place.

ICNIRP (International Commission for Non-Ionizing Radiation Protection) is an

independent organization, which provides scientific advice and guidance on the health

and environmental effects of non-ionizing radiation (NIR) to protect people and the

environment from detrimental NIR exposure. Table 4.6 shows the exposure values

published by them [20];

Table 4.6 - EMF exposure limits

Electric Filed (kV/m) Magnetic Field (μT)

Public 5 100

Occupational 10 500 Source: http://www.icnirp.org/

These are the values that most of the utilities in the world are adhered to. In Sri Lanka

also, there are no any country specified values on restricting EMF exposure under

overhead lines and therefore values publish by ICNIRP are used. Typically electric

field under the power line depends on the voltage and the magnetic field depends on

current flowing in the line.

After considering above two factors, it is obvious that the improvement of tower safety

factors by reducing tension is not a solution as the safety clearances are getting violated

with the increase in conductor sag. Therefore according to the algorithm, the next

option shall be considered.

48

4.3.2 Reconstruction with the use of Design Software – PLS CADD

Without a doubt, PLSCADD has become the most premium software package used for

overhead line design in the world. Therefore, the same transmission line (Pannipitiya

– Rathmalana 132kV line) is studied using PLSCADD software for its competency to

be upgraded based on the algorithm given above [21]. In Appendix D, complete PLS

Design is given.

1. Profile Data

Ground coordinate shall be provided in terms of Latitude and Longitude with elevation

data, to form a profile of the transmission line route in PLSCADD. Therefore it is

always useful to have these data available for the transmission line to be studied. If

those data are not available, there are various methods to develop the profile of the

line.

• Carrying out a complete ground survey of the line

• Carrying out a LIDAR survey

• Use of Google Earth data

Carrying out a ground survey will require extra amount of time as well as man power.

However ground survey data are more accurate compared to other methods given

above. LIDAR is a technology that is used for remote sensing and the same have

developed in a way that it could be used for transmission line surveying. This requires

an air borne flat form, typically a helicopter or a fixed wing aircraft. This method is

bit costly and has never been used in Sri Lanka.

Easiest method of developing ground profile is the use of Google Earth data. Google

Earth is freeware which popular all around the world as a virtual globe. These data can

be extracted with the use of online software and could be converted into preferable

geographical coordinate systems. Though the accuracy of google earth data is low

compared to ground survey data, they are more than enough for the preliminary studies

of developing ground profile. Therefore in this study, Google Earth coordinates were

extracted to develop the ground profile of Pannipitiya – Ratmalana overhead line.

49

2. Feature Code

To develop a line route in PLSCADD, there are minimum set of requirement that shall

be input to the programme. From google earth software, we can grab x,y,z (Longitude,

Latitude, Elevation) data. Then these set of data have to be assigned with a code called

feature code data. Below are the feature code data that have been used for the design;

200 - Ground Points

100 - Angle Points

There are other feature code data such as roads, rivers, tanks, buildings could be

defined during detailed designed stage of the line. However, initial study shall be done

using above two feature code data.

2. Criteria Files

Figure 4.10 - Feature Code View

Figure 4.11 - Weather Criteria File

50

When developing an overhead line design using PLSCADD, it is required to develop

criteria files (Files that keep design inputs) at the beginning of the programme. This

includes safety factors, environment data, cable types, weather data etc.Maximum

operating temperature of Pannipitiya – Ratmalana line is 54°C and the minimum

operating temperature of the line is 15°C. Therefore the temperature criteria have been

designed to be matched with the existing line design.

3. Structure Files

Structures files can be developed using PLSCADD/ PLSTOWER software based on

“As built drawings” or ground surveyed measurements.

4. Cable Tension and Automatic sagging

Cable tensioning and sagging of conductors have been done according to the safety

limits published under CEB technical Specifications given in Table 3.7.

5. Stringing/ Sagging

Conductor stringing could be done in two ways;

a. Based on the measured/ surveyed sag values

Conductor ground clearance and temperature could be measured using hot line

tools and with use of thermal recoding equipment. Conductor maximum sag

values can then be calculated and conductors will be strung according to the

calculated values using PLSCADD. This method is more accurate, when the

design criteria of the existing line is not known.

b. Based on Automatic Sag

Figure 4.12 - Automatic Sagging Criteria

51

Conductors could be strung using automatic sag option in PLSCADD. If the

initial line was designed based on the correct design criteria, this method is

useful and it can reduce labour hours of collecting ground survey

measurements. Pannipitiya- Ratmalana transmission line is modelled using

method b.

Figure 4.13 shows the stringing details of the transmission line. Here it could

be seen that the catenary value is around 2050m.

Catenary constant (C) =

𝐻𝐻𝑊𝑊

= TensionConductor Unit Weight

= 83.1×1000𝐴𝐴.𝐹𝐹 ×8.289

=2050m

Safety Factor@ EDS = 4.89 (> 4.5)

It can be seen that the conductor tension is satisfying the safety requirement at EDS

condition. Stringing is done at 32°C. Output display is selected to be showing the

Figure 4.13 - Section Modify window

52

conductors at its maximum temperature. Figure 4.14 shows the simulated profile view

of Pannipitiya – Ratmalana 132kV existing Lynx line.

Figure 4.14 - Profile view of Panniptiya –Kolonnawa ACSR Lynx line

Use of ACSR Zebra conductor in the same towers

It was observed that there is no ROW for a new transmission line to be constructed

and according to the algorithm it was chosen to upgrade the existing line by

introducing a new ACSR conductor with a higher cross section.

Figure 4.15 - Section Modify window for Zebra conductor

53

Therefore the same software design was carried out for Zebra conductor using selected

operating condition as in the case of Figure 4.3 above.

Here, Operating Temperature is selected as 75°C and minimum temperature was taken

as 7°C to be matched with present design requirements.

Horizontal Tension of the conductor = 27730 N

Safety Factor @ EDS Condition

= UTS of Lynx Conductor where existing towers are designedTension of Zebra @ EDS

= 83.1×100027726.7

= 3 (<4.5)

It can be seen that, with the given criteria, the safety factor could be achieved is less

than 4.5 which is the required safety factor based on CEB specifications.

Figure 4.16 shows the profile view of Pannipitiya – Ratmalana 132kV line with Zebra

conductor.

It can clearly be seen that even at this tension, the conductor violates clearance curve

in many sections of the line. Therefore reduced tension to achieve more safety will not

be doing any good as the sag increases with reduced tension.

Checking EMF Level under the power line

Using PLSCADD design software, EMF field study has been carried out and the

graphs showing electric and magnetic field level are prepared below.

Figure 4.16 - Profile View of Pannipitiya-Ratmalana 132kV Zebra line

54

Electric Fields

Magnetic Field

Figure 4.17 - (a) Electric Field of Panniptiya – Ratmalana Existing Lynx Line

Figure 4.18 - (b) Electric Field of Pannipitiya – Ratmalana Upgraded Zebra line

Figure 4.19 - (a) Magnetic Field of Panniptiya – Ratmalana Existing Lynx Line

Figure 4.20 - (b) Magnetic Field of Panniptiya – Ratmalana Upgraded Zebra Line

55

Table 4.7 - EMF comparison

Field under existing

Lynx Line

Field under uprated

Zebra line

Electric Field 3.2kV/m 6.4kV/m

Magnetic Field 12.2μT 45μT

Figures 4.17, 4.18, 4.19 and 4.20 show the EMF level at the middle point between

tower No 7 and 8 of Pannipitiya-Ratmalana line before and after it is uprated. Table

4.7 shows the values of electric and magnetic fields of each case and it can clearly be

seen that the electric field value has been risen beyond the allowed level (See Table

4.6) once the line is uprated. Though there is an increase in the magnetic field, it is still

not harmful according to the limits published by ICNIRP. This has happened due to

the increased sag of Zebra conductors at the operating temperature of 75°C.

It was seen above that the safety factor of the line is around 3.0 which is below the

required value of 4.5 at EDS condition. However according to the algorithm, there is

no need to go to the next option of reduction of tension of the conductor to increase

safety limits as EMF values are already have exceeded the boundary level. Therefore

the only option left in this case is to go for reconductoring using HTLS conductors.

Summary Flow Chart of Pannipitiya – Ratmalana existing line uprating case

study

Table 4.8 shows the path for selecting the most appropriate solution in the case of

Panniptiya – Ratmalana 132kV line uprating based on the algorithm given under the

chapter 2.2.3.

56

Table 4.8 - Summary flow chart of the case study of Pannipitiya- Ratmalana line

uprating.

Condition Check for Result Comment

Availability of

ROW Rural Area No

Line is located in a heavily

populated area and there is no

ROW availability between

respective substations

Use of Larger

ACSR conductor

in the same line

Adequate

tower strength No

Existing towers are not capable of

absorbing additional forces exerted

by new ACSR conductor without

violating safety factors

Reducing the

tension of ACSR

conductors

Safe EMF

level No

Further reduction of tension will

cause conductors to sag more and

in turn will increase the EMF level

under the power line beyond its

safe limits

During the above case study, most of the factors in the algorithm were discussed that

involves in the path above. However it is important to discuss other options that could

be resulted in the process of selecting the best solution in line upgrading. As an

example, in a case where there is enough ROW availability in between substations to

construct a new transmission line, the decision has to be taken based on economic

considerations.

Case 2: Samanalawewa - Embilipitiya 132kV transmission line

This line is proposed to be uprated according to the long time transmission planning

programme of CEB. The line was constructed with the use of ACSR Lynx conductor

and supposed to be thermally uprated to have a current carrying capacity similar to

Zebra conductor. This is similar to the case of Pannipitiya – Ratmalana transmission

line discussed above.

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If the same algorithm is used in this case, it is seen that there is enough ROW near the

existing power line to construct a new line unlike in the case of Pannipitiya –

Ratmalana line. The line is basically running in an area where there is neither much

population nor pile of constructions. However attention shall be given to

environmental importance of the area as there are few forest reserves located in the

area.

According to the algorithm, it is seen that the availability of ROW leads to checking

of economic feasibility of the new line. Economic feasibility shall be studied under

different perspectives and this require various approaches.

Economic Feasibility will depend on below factors;

Table 4.9 - Factors to be considered for Economic Feasibility

Uprating an

Existing Line

Construction of New

Line

Cost of major equipment Only conductor cost Total Project cost

Compensation for crop

damage No

Crop damage shall be

given

Line interruption cost Require interruption No need of interruptions

Conductor energy Loss High at higher

temperatures Low

Environmental considerations Very Low High

Economic feasibility of reconstruction and uprating will be discussed in the next clause

under selection of HTLS conductors.

4.3.3 Selection of HTLS Conductors for Restring

It is seen that in the above case studies, uprating of existing Panniptiya – Ratmalana

transmission line can only be done with the use of HTLS conductors. The next

challenge is to find out the most suitable HTLS conductor, as there are few number of

various HTLS conductors are available in the market.

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Major Factors to be considered when selecting the most appropriate HTLS conductors.

• Similar Dimension as with the existing Lynx conductor, so that it will not affect

the transverse forces being exerted on the line

• Similar unit weight, so as to keep similar vertical forces.

• Similar UTS, so that towers could be tension without violating safety factors.

• Lower KPT, so that conductor sag will increase at a very low rate, from the

very beginning.

• Lower unit resistance, so that the conductor I2R losses could be restricted, at

higher operating temperatures.

Pannipitiya – Ratmalana line is to be uprated to have double the capacity of Lynx

conductor. CCC of Lynx conductor at 54°C is 400A. Therefore, selected HTLS

conductor shall have double the capacity of Lynx conductor and its sag value shall not

violate the required ground clearance at the particular temperature. At the same time,

conductor tension on conductors shall be similar so that no need of tower

modifications.

UTS of Lynx Conductor = 79.8kN

Safety Factor at sagging condition = 4.5

Tension on towers = 79.8/4.5 = 17.73kN

Table 4.10 - Conductor Stringing Tensions

Conductor Type ACCC GTACSR ZTACIR ACSS

Conductor Name Oriole 200mm2 159-160 Lark

UTS 98.3 80 63.7 77.8

Safety Factor 5.54 4.51 3.59 4.39

% RTS 18.05 22.17 27.86 22.78

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Table 4.11 - Properties of HTLS conductors

Conductor Type ACCC GTACSR ZTACIR ACSS

Conductor Name Oriole 200mm2 159-160 Lark

Diameter (mm) 18.821 19.0 18.2 17.781

Cross Section (mm2) 222.3 208 159.3 201.4

Unit Weight (kg/km) 688.9 844.8 706.8 925.3

Operating Temp. when CCC

is 800A (°C) 114 140 173.5 147.8

Unit Resistance at given

operating Temp. (Ω/m) at 0.12831 0.21088 0.27101 0.20721

KPT (°C) 70 32 117 98

Sag @ KPT (m) 5.6 6.14 7.03 7.98

Sag @ operating Temp. (m) 5.72 7.81 7.33 8.84

Annual Energy Loss (MWh) 15,774 18,720 23,295 17,810

From Table 4.11, it could be seen that ACCC conductor provides the best

performances as it gives the lowest losses and the lowest sag value. At the same time,

ACCC provides higher safety factors during stringing which is 5.54 (>4.5) compared

to other types of conductors.

Sag values given by ACSS conductor exceeds the maximum allowable sag value of

Lynx conductor 54°C, which is 7.72m. Though ZTACIR conductor has slightly higher

sag, this could be reduced by increasing the tension by a small percentage. Therefore,

ACCC, GTACSR and TACIR conductors could achieve required clearance levels.

Though, ACCC provides the best performances in terms of losses and mechanical sag,

it requires special string methods compared to conventional stringing. GTACSR

conductor also requires special two method string and require trained staff. On the

other hand, ZTACIR conductor can be strung with conventional methods and no need

of trained staff for stringing. Stringing is further described in chapter 6 of this

document.